Device for the concentration of vibrational energy



Au H, mm M. LAST ETAL 3,524,033

DEVICE FOR THE CONCENTRATION OF VIBRATIONAL ENERGY Filed Dec. 16, 1968 2Sheets-Sheet 1 67 PDT/i 65 65 "45 47 ANTHONY SYEET 5 MI HAEL (NM-N.)KlSLY' FIG. BY 47 6% Aug. 11, 1970 J, T ET AL 3,524,083

DEVICE FOR THE CONCENTRATION OF VIBRATIONAL ENERGY Filed Dec. 16, 1968 2Sheets-Sheet 2 AR /BR w 4 A w M W L P 8 A R :Q m a. Q A 2 mw M F A4 1M.9

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INVENTORS: 1

ANTHONY J- LAST United States Patent Oifice 3,524,083 Patented Aug. 11,1970 3,524,083 DEVICE FOR THE CONCENTRATION OF VIBRATIONAL ENERGYAnthony J. Last, Oakville, Ontario, and Michael Kisly,

Toronto, Ontario, Canada, assignors to Ontario Research Foundation,Toronto, Ontario, Canada Filed Dec. 16, 1968, Ser. No. 783,942 Int. Cl.H01v 7/00 US. Cl. 3108.1 14 Claims ABSTRACT OF THE DISCLOSURE A devicewhich utilizes a parabolic interface to concentrate initially parallelvibrations at a single point or line, corresponding to the focus of theparabolic interface. A transducer sandwiched between two solid elementsis used to generate vibrations emanating in opposite directions throughthe two solid elements, and the solid elements are dimensioned such thatany portion of the vibrations reflected back to the transducer arrivesin phase with the transducers motion. A passageway is provided in one ofthe solid elements through the focus of the parabolic surface, suchthat, for example, two liquids can be passed through the passageway andemulsified or mixed by the concentrated vibrations.

This invention relates generally to devices for concentratingvibrational energy in a small volume, and has to do particularly withdevices adapted to cause violent agitation of a liquid passed throughthe vicinity of energy concentration. Such agitation could be used, forexample, to emulsify two immiscible liquids, or to effect the dispersionof a solid in a liquid.

GENERAL BACKGROUND OF THE INVENTION It is well known to utilizeultrasonic vibrations to cause emulsification or to disperse solids in aliquid. The following patents may be referred to in this connection:U.S., 2,606174, Aug. 5, 1952, Kolthoif et al.; U.S., 3,165,299, Jan. 12,1965, Balamuth et al.; German, 712,216, Oct. 14, 1941, Hertz et al.;German, 716,231, Jan. 15, 1942, Hertz et al.; German, 960,893, Mar. 28,1957, Hertz; German, 718,744, Mar. 19, 1942, Sch'cifer; German, 994,667,June 21, 1956, Sauter.

Those skilled in this art will be familiar with the various methods ofgenerating ultrasonic vibrations. These methods may be classified aselectrostrictive, magnetostrictive, or mechanical. The present inventionis particularly adapted to utilize either the electrostrictive or themagnetostrictive method of vibration generation. Of the two types ofelectrostrictive elfects known as piezoelectric and ferroelectric, it isthe former effect which is preferably used in the electrostrictiveapplication of this invention. The piezoelectric effect is a phenomenonexhibited by materials such as quartz, Rochelle salt, and tourmaline,wherein the crystal changes its length over certain crystallographicaxes by a differential varying directly with an electric field placedacross the crystal. Thus, a high-frequency alternating electric fieldplaced across the appropriate axis of a crystal of quartz will cause thecrystal to vibrate at the same frequency as that of the electric field.Generally, natural crystals have been replaced in modern practice byartificial crystals, and such materials as barium titanate and leadzirconate titanate can be made to act in a piezoelectric manner byprepolarization.

The magnetostrictive types of generators are based on changes is shapethat occur when certain substances are magnetized. Nickel andnickel-copper ferrites are magnetostrictive materials. When rods madefrom these substances are magnetized by sending a high-frequencyalternating current through coil windings around them, the length of therods varies with the changes in polarity, causing them to oscillate withthe applied current. Just as in the case of electrostrictive materials,there are two magnetostrictive types. Those producing deflectionsdirectly proportional to the magnetic field are demonstrating thepiezomagnetic effect, and this effect is preferably used in themagnetostrictive application of this invention. In order to act in apiezomagnetic manner, a bias polarization must be maintained on thematerial and this in fact is done for all power applications.

Devices such as the above, capable of transforming one form of energyinto another form, are called transducers.

In the adaptation of the above effects to accomplish the objects of thisinvention, a sandwich construction is utilized throughout, wherein atransducer, either electrostrictive or magnetostrictive, is tightlysandwiched between two blocks, the latter usually of metal. Thisconstruction permits a selection of the resonance frequency at whichefliciencies are highest.

OBJECTS OF THE INVENTION One object of this invention is to provide adevice for concentrating vibrational energy capable of generating andfocusing high-frequency mechanical vibrations at a single point or alonga single line.

A further object of this invention is to provide a device capable ofviolently agitating a liquid, by focusing highfrequency mechanicalvibrations in the liquid at a single point or along a single line.

GENERAL DESCRIPTION OF THE INVENTION Generally, the invention provides adevice consisting of a sandwich in which a transducer, eitherelectrostrictive or magnetostrictive, is firmly sandwiched between twoblocks of metal in such a way that longitudinal mechanical vibrationscan be propagated rectilinearly in mutually opposed directions away fromthe transducer into the two blocks of metal. At least one of the blockshas a convex parabolic surface of which the axis is parallel to thedirection of vibration propagation, and this parabolic surface isadapted to concentrate the longitudinal vibrations at the focus of theparabolic surface. A passageway is provided in the block with theparabolic surface, the passageway containing the focus of the parabola.

The preceding paragraph sets out the basic proposition from which thisinvention proceeds: namely, the concentration of parallel longitudinalvibrations by focusing the said vibrations through reflection at aparabolic surface. Further investigation of the vibrationalcharacteristics of the concentrating device, however, indicates that aportion of the vibrations converging at the focus of the parabolareflects off the passageway wall, reflects again from the parabola, andreturns to the transducer. Also, in addition to the mechanicalvibrations emanating directly from the interface between the transducerand the metal block having the parabolic surface, the transducer emitsinto the other metal block vibrations which could likewise be reflectedback to the transducer having encountered a surface or surfaces of theother metal block. If any of the reflected vibrations were to arrive atthe transducer at an angle to the original vibration direction, orparallel to the original vibrations but out of phase with thetransducer, interference would take place at the transducer and theefficiency of the device would suffer.

In view of the above, this invention further provides a constructionwhereby any portion of the mechanical vibrations generated by thetransducer which return after reflection to the transducer are in phasewith the latter, or substantially so, in Order to maximize theefficiency of the device.

More specifically, this invention provides a device for concentratingvibrational energy, comprising: a transducer sandwiched between a firstand a second solid element, the first solid element having a first planesurface in contact with said transducer and a convex parabolic surfaceof which the parabolic axis is normal to said first plane surface, thedirectrix of said parabolic surface being parallel to said first planesurface, a passageway in said first solid element through which a fluidmay be transmitted, said passageway containing the focus of saidparabolic surface, the second solid element having a second planesurface in contact with said transducer, the transducer being adapted totransmit vibrations into said first solid element in the directionnormal to said first plane surface, said vibrations being reflected fromsaid parabolic surface to converge on said passageway, a first portionof said vibration passing through the wall of said passageway into thefluid, a second portion of said vibration being reflected from the wallof said passageway and then reflected from the parabolic surface towardsaid first plane surface in the direction normal to said first planesurface, said first solid element being dimensioned such that saidsecond portion arrives at said transducer in phase therewith, thetransducer also transmitting further vibrations into said second solidelement in the direction normal to said second plane surface, saidfurther vibrations being generated simultaneously with saidfirst-mentioned vibrations, said second solid element being dimensionedsuch that any reflected part of said further vibrations returning to thetransducer arrives at said transducer in phase therewith.

GENERAL DESCRIPTION OF DRAWINGS Eight embodiments of this invention areshown in the accompanying drawings, in which like numerals denote likeparts throughout the several views, and in which:

FIG. 1 is a perspective view of one form of the first embodiment of thisinvention;

FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1;

FIG. 3 is an axial sectional view of another form of the firstembodiment of this invention;

FIG. 4 is an axial sectional view of yet another form of the firstembodiment of this invention;

FIG. 5 is a cross-sectional view taken at line 5-5 in FIG. 3; and

FIGS. 6-13 inclusive are simplified diagrams representing axialsectional views, respectively, of the eight embodiments of thisinvention.

Reference is made first to FIG. 1, in which an emulsifying device 10 isseen to include a first solid element 12, and a second solid element 14.The word solid is used in this context to distinguish solid from liquidor gas, and does not necessarily mean that the elements 12 and 14 arenot cavitied. In the preferred form of the first embodiment shown inFIGS. 1 and 2, the second solid element 14 is in fact free of cavities,but the first solid element contains a passageway 16, later to bedescribed in detail. The second solid element 14 has-an upper planesurface 18, and the first solid element 12 has a lower plane surface 19.The surfaces 18 and 19 bear against the opposite surfaces of anelectrostrictive transducer 22, which is preferably a piezoelectriccrystal such as pre-polarized lead zirconate titanate.

The lower solid element 14 is in the shape of a rectangularparallelepiped. Referring to the upper solid element 12, the lower planesurface 19 and the four vertical walls are those of a rectangularparallelpiped, but opposite the plane surface 19 is a convex parabolicsurface 24 of which the parabolic axis 25 (see FIG. 2) is normal to theplane surface 19. The directrix 26 of the convex parabolic surface 24 isa plane, and is parallel with the plane surface 19. As best seen in FIG.1, the convex parabolic surface 24 is a cylindrical surface. The wordcylindrical is here used in its broadest since, i.e.

to define a surface which is the locus of a straight line movingcurvilinearly in a direction normal to its length. Thus, allintersections of the parabolic surface 24 with planes parallel to thevertical section plane of FIG. 2 are identical.

As is well known, a parabolic curve is defined as the locus of a pointmoving so as always to be equidistant from a straight line and from apoint not on the straight line. Looking at FIG. 2, the directrix 26 isthe straight line" in the foregoing definition (actually a plane due tothe cylindrical characteristic of the parabolic surface 24), and a pointshown at 28 is the point in the definition. It is also known that areflective parabolic surface has the property of reflecting to a singlepoint longitudinal mechanical vibrations travelling parallel with theparabolic axis. The point upon which the reflective rays converge is thesame point as that mentioned in the definition of the parabolic surface,and will be hereinafter referred to as the focus 28 of the parabola 24.The reflection phenomenon at a given surface is due to a great acousticimpedance mismatch between the solid element and the gas (in this caseair) contacting the surface.

Attention is again directed to the piezoelectric crystal system(transducer) 22, as seen in FIG. 2. The piezoelectric crystal system 22is adapted to be excited by a highfrequency electric field produced by agenerating device of conventional nature shown schematically at 30. Theelectrical drive connections to the crystals 22a and 22b, which arecarefully matched, follow normal techniques which are well known tothose skilled in the art to which this invention pertains. Multiplecrystal systems may also be used, or a magnetostrictive transducer canalso be substituted.

When the generating device 30 applies an alternating electric fieldsymmetrically across the two crystals 22a and 22b of the transducer 22(synonymous with crystal system 22), it causes these crystals to expandand contract in the axial direction in phase with the alternatingelectric field. The resulting mechanical vibrations are transmitted tothe solid elements 12 and 14, and travel through them in a directionperpendicular to the plane surfaces 18 and 19 at a speed determined bythe physical characteristics of the material from which the solidelements 12 and 14 are constructed.

Looking particularly at the vibrational disturbance A in FIG. 2, thisdisturbance is shown to travel upwardly along the path 35, and reflectoff the parabolic surface 24, a metal-air interface, along the line 36in the direction of the focus 28. The A-path is the direct route takenby vibrational disturbances emanating from the upper portion 22a of thecrystal and converging on the focus 28. The A-path involves a singlereflection at the parabolic surface 24, and this reflection results in aphase shift for the disturbance A.

If the passageway 16 is filled with liquid and the solid element 12 ismade of steel, about 88% of the vibrational energy converging on thefocus 28 is reflected at the metalliquid interface constituted by thewalls of the passageway 16. Lower reflection percentages would beencountered if the solid element 12 were made of another material withbetter acoustic matching characteristics. The reflected portion of theenergy converging at the focus 28, denoted by A travels back alongsubstantially the same path, because it is preferred that at least theupper surface of the passageway 16 be cylindrical, with its axiscoincided with the focus 28. The reflected portion A of the energytravels back to the transducer 22, and is in phase with and reinforcesthe negative disturbance emanating from the transducer one-halfwavelength later. If the reflected portion A does not return to thetransducer 22 in phase with the transducer, interference will take placewith a resultant loss of efliciency.

There is also generated a mechanical vibration travelling initiallydownwardly in the solid element 14 from the upper plane surface 18. Thispath is denoted by the letter B. The B-path involves a reflection fromthe remote plane surface 38 of the solid element 14, and vertical travelback to the transducer 22. It is, of course, also necessary that the Bdisturbance arrive at the transducer 22 in phase with the latter, inorder to maximize efiiciency.

Each of the eight embodiments of this invention constitutes a differentway in which the in-phase arrival of reflected vibrations at thetransducer 22 can be effected.

In the first embodiment of this invention, shown in FIGS. 1 and 2, the Aand B disturbances are made to arrive at the transducer 22 in phase withthe latter by dimensioning the device 10 in such a way that thetransducer 22 is spaced from the surface 38 by a distance representingA/4, and is spaced from the directrix 26 of the parabolic surface 24also by a distance representing A/4, where APrepresents the wavelengthof mechanical vibrations in the device 10. It has been found that asatisfactory mathmatical model of the physical device can be representedin the manner of FIG. 6, wherein the thin transducer crystal has beenreduced to a line of zero thickness, labelled TR.

The in-phase arrival at the transducer 22 of disturbances A and B inFIG. 2 is established as follows. Disturbance A considered to originate.at the transducer surface, travels to and reflects from the parabolicsurface 24, reflects from the surface of the passageway 16, againreflects from the parabolic surface 24, and returns to the transducer22, having travelled a total distance of A/2. Three phase-reversalsoccur at the three reflections, and their distance equivalent is 3A/ 2.Thus, the effective acoustical path length for disturbance A isDisturbance B emanating downwardly from the lower transducer surfacecovers a distance A/ 4, is reflected once, and returns to the transducer22, having travelled a distance of A/ 2. One phase-reversal occurs atthe surface 38, such that the acoustical path length for disturbance Bis It is thus seen that, in order for reflected disturbances to returnto the transducer 22 in phase with the latter, the effective acousticalpath length must be a multiple of a whole wavelength.

It will also be clear from the above discussion that the in-phasearrival at the transducer 22 of disturbances A and B will also beassured if the distance from the transducer to either the directrix 26or the surface 38, or both, is increased by some multiple of one-halfwavelength. For example, increasing this distance to 2 AA would notdisturb the in-phase arrival at the transducer 22 of disturbances A andB.

In the first embodiment of this invention, shown in FIGS. 1 through 5,the force maintaining the solid elements 12 and 14 in pressure contactwith the transducer 22 is provided by four bolts 40 which extend looselythrough appropriate boreholes in the lower solid element 14, and whichare tightly threaded into tapped bores in the upper solid element 12extending upwardly from the plane surface 19.

Turning now to FIG. 3, a second form of the first embodiment of thisinvention is shown, in which the parabolic surface 42 is a paraboloid ofrevolution, having a single point focus 44, rather than a linear focusas does the first form of this invention shown in FIGS. 1 and 2. Becausethe surface 42 is a paraboloid of revolution, the section -5 iscircular, as seen in FIG. 5. The device shown in FIG. 3 is likewiseprovided with a first solid element 45, a second solid element 46, andbolts 47 securing the two solid elements 45 and 46 in sandwichingrelation to a piezoelectric crystal 49, the latter being identical inits function with the piezoelectric crystal shown in FIGS.

1 and 2.

Whereas in FIGS. 1 and 2 the liquids to be emulsified are passed alongpassageway 16 parallel with the plane surface 19 of the solid element12, in the FIG. 3 form of this invention, a central, axial passageway 50is provided for the liquids to be emulsified, the passageway 50extending upwardly from the lower surface 52 of the solid element 46,centrally through the piezo electric crystal 49 (which is suitablybored), upwardly through the solid element to encompass the focus 44,and from there a reduced extension 54 of the passageway extends to theupper, central point of the surface 42. The liquids to be emulsifiedenter the bottom and the emulsified mixture is ejected at the end of theextension 54.

FIG. 4 shows another form of the first embodiment of this invention,which consists of the same basic components as the devices shown inFIGS. 13. An upper solid element 56 and a lower solid element 58 aretightly secured together in sandwiching relation about a transducer 60by a single bolt 62 which extends loosely through an appropriately sizedbore-hole in the lower solid element 58, and is tightly threaded into atapped bore 63 in the upper solid element 56. Two or more radialpassages 65 extend inwardly and communicate with a central cavity 66which contains the focus 67. The cavity 66 is merely the termination ofthe bore 63. A vertical passageway 68 extends upwardly from the cavity66. The liquids to be emulsified are introduced at the radial passages65, travel inwardly to be emulsified at the focus 67, and are ejected asan emulsion upwardly through the passage 68. The parabolic surface 70 inFIG. 4 is also a paraboloid of revolution.

The passageway 16, the upper termination of the passageway 50 and thecavity 66 are preferably all curved so as to be normal to the convergingdisturbance A as the latter is reflected toward the foci of the threeforms by the respective parabolic surfaces. If this were not the case,the portion of the distribance being reflected from the liquid-metalinterface would not retrace its original path, and would not necessarilyreach the transducer 22 in phase. This is particularly important wherethe passageway is of substantial width. Where the passageway width isvery small, it will be appreciated that reflection therefrom in randomdirections will not have a serious effect on the angles at which thevibrations pass through the crystal 60. Generally speaking, however,where the focus is a straight line as in FIG. 2, it is preferable thatat least the upper portion of passaegway 16 be cylindrical with thefocus at the cylindrical axis. The lower portion can be flat as shown,whenever the focus is not within the volume defined by the parabolicsurface, since in this case the converging vibrations do not impingeupon the lower side of the passageway. Where the focus is a singlepoint, as in the forms shown in FIGS. 3 and 4, it is preferable that atleast the upper portion of the passageway be spherical with the focus atthe centre of spherical curvature.

In FIGS. 3 and 4, the transducers 49 and 60 are excited in exactly thesame way as the piezoelectric crystal in FIG. 2. This has not beenshown, because it is conventional.

Attention is now directed to FIGS. 6 through 13, showing the eightseparate embodiments of this invention. The first four embodiments shownin FIGS. 6, 7, 8 and 9 all utilize electrostrictive crystals forvibration generation, and since these are small compared to onewavelength, they are represented as a single, hypothetical plane ofvibration generation, and are denoted by the letters TR. The remainingembodiments of this invention, shown in FIGS. 10, 11, 12 and 13, utilizemagnetostrictive transducers, and these are always one-half wavelengthlong, and are designed for a specific resonance frequency. For thisreason, the transducer is represented in FIGS. 10 through 13 as a blockof material whose length is equal to one-half wavelength. It will beappreciated that the speed of transmission through the magneto- 7strictive transducer is not necessarily the same as the speed oftransmission through the solid elements at either end of themagnetostrictive transducer, and that the actual length of thetransducer, and of the solid elements, is a function of the speed oftransmission. In the embodiments shown in FIGS. through 13, the entiremagnetostrictive transducer block is labelled TR. FIGS. 6 to 13 showonly the paths of the reflected vibrations.

FIG. 6-EMB ODIMENT 1-ELECTROSTRICTIVE Effective acoustical path lengthLA =%+%=2A FIG. 7-EMBODIMENT 2-ELECTROSTRICT IVE In this embodiment bothof the solid elements have a parabolic surface remote from thetransducer TR, and the following formulas will show that, at thetransducer TR, the two disturbances arrive substantially in phase.

(B is the reflected portion of disturbance B.)

FIG. 8-EMBODIMENT 3ELEC1ROSTRICTIVE It is possible to insert transitionlayers of one-quarter wavelength or one-half wavelength between thetransducer and the solid element containing the parabolic surface andthe focus of the latter. These one-quarter wave or one-half wave studsmay be considered as transmission lines which can be constructed on thebasis of transmission line theory to provide matching of high to lowimpedances.

(a) Ignoring internal and surface reflectivity losses, a half-wave stubadds a resonant section to the resonant crystal and does not change thephase relationships in the creation of a standing wave condition. Itacts as though it is not present, i.e. the resonant crystal (ormagnetostrictive transducer) sees as its load only the medium whichloads the half-wave stub.

(b) A quarter-wave stub, however, acts as an acoustic impedancetrannsformer. Applying transmission line equations, we get eventuallywhere Zi is the acoustic impedance seen looking into a transmission lineof characteristic acoustic impedance Zm and where Zm= /ZlZt. Zl is theload impedance and Zt is the transducer impedance. In other words, it ispossible to transform the impedance of the loading medium in such a waythat the crystal sees a perfect match. The selection of transmissionlines is very im' portant and has a direct effect on the amount of soundenergy reflected at the metal-liquid interface constituted by the wallof the passageway.

Naturally, a number of layers or stubs may be used for matching, but theincreased absorption with extra layers must be balanced with theincreased transmission. This absorption is due to both the extrainterfaces and the extra material in the transducer.

Turning to FIG. 8, the transducer is represented as TR, on the right ofwhich is a quarter-wave solid element 72, and on the left of which is,first, a quarter-wave stub 74, and then a one-wave element 76 having aparabolic surface 78.

8 The following formulas apply to the third embodiment of thisinvention, shown in FIG. 8:

FIG. 9EMBODIMENT 4-ELECTROSTRICTIVE In this embodiment, the device issymmetrical about the transducer TR, and consists of two quarter-wavestubs adjacent the transducer, and two half-wave parabolic elements 82fixed to the remote surface of the quarterwave stubs 80.

In this embodiment, the following equations apply:

FIG. 10EMBODIMENT 5MAGNETOSTRICTIVE FIG. 11EMBODIMENT6--MAGNETOSTRICTIVE In this embodiment, two parabolic solid elementssandwich between them the magnetostrictive transducer 92.

The applicable formulas are as follows:

FIG. 12EMBODIMENT 7MAGNETOSTRICTIVE In this embodiment, a quarter-wavesolid element 93 is fixed to the face E of the transducer, aquarter-wave stub 94 is fixed to the face F of the transducer, and ahalfwave element 96 hearing the parabolic surface and the focalpassageway is in turn fixed to the quarter-wave stub 94.

The applicable formulas are as follows:

7\ E.A.P.L. LB -l- -k FIG. 13--EMBODIMENT 8MAGNETOSTRICTIVE Thisembodiment is symmetrical about the transducer. A quarter-wave stub 98is fixed to each face of the transducer, and a half-wave element 99 isin turn fixed to each quarter-wave stub 98.

The applicable formulas are as follows:

Generally speaking, the higher is the effective acoustic path length,the greater is the absorption of energy by the device. Practicallyspeaking, this limits the system to one, or at most three, quarter-wavestubs.

While preferred embodiments of this invention have been disclosedherein, those skilled in the art will appreciate that changes andmodifications may be made therein without departing from the spirit andscope of this invention as defined in the appended claims.

What we claim as our invention is: 1. A device for concentratingvibrational energy comprising:

a transducer sandwiched between a first and a second solid element, p

the first solid element having a first plane surface in contact withsaid transducer and a convex parabolic surface of which the parabolicaxis isnormal to said first plane surface, the directrix of saidparabolic surface being parallel to said first plane surface,

a passageway in said first solid element through which a fluid may betransmitted, said passageway containing the focus of said parabolicsurface,

the second solid element having a second plane surface in contact withsaid transducer,

the transducer being adapted to transmit vibrations into said firstsolid element in the direction normal to said first plane surface, saidvibrations being reflected from said parabolic surface to converge onsaid passageway, a first portion of said vibrations passing through thewall of said passageway into the fluid, a second portion of saidvibrations being reflected from the wall of said passageway and thenreflected from the parabolic surface toward said first plane surface inthe direction normal to said first'plane surface, said first solidelement being dimensioned such that said second portion arrives at saidtransducer in phase therewith,

the transducer also transmitting further vibrations into said secondsolid element in the direction normal to said second plane surface, saidfurther vibrations being generated simultaneously with saidfirst-mentioned vibrations, said second solid element being dimensionedsuch that any reflected part of said further vibrations returning to thetransducer arrives at the transducer in phase therewith;

2. A device as claimed in claim 1, in which said parabolic surface iscylindrically parabolic, such that the focus is a straight line parallelwith said first plane surface, said passageway being rectilinear andcontaining said straight line defining the focus.

3. A device as claimed in claim 1, in which said parabolic surface is aparaboloid of revolution, such that the focus is a single point, theportion of the passageway containing said single point having aspherical surface remote from said first plane surface, the single pointlying substantially at the centre of curvature of said sphericalsurface.

4. A device as claimed in claim 1, in which said second solid elementhas a further plane surface parallel with said second plane surface andopposite thereto.

5. A device as claimed in claim 1, in which the length of the effectiveacoustic path for vibrations leaving and returning to the transducer ineither solid eleinent, calculated as the sum of the actual geometricdistance covered plus the distance equivalent of reflection phasechanges, is one wavelength or a multiple of one wavelength.

6. A device as claimed in claim 4, in which the transducer iselectrostrictive and is thin compared with said solid elements, wherebya hypothetical plane of vibration generation can be postulated betweenthe two solid elements, the distance between said hypothetical plane andsaid further plane surface being equal to one-quarter of the wavelengthof said vibrations, the distance between said hypothetical plane and thedirectrix of said parabolic surface being equal to one-quarter of thewavelength of said vibrations.

7. A device as claimed in claim 4, in which the transducer iselectrostrictive and is thin compared with said solid elements, wherebya hypothetical plane of vibration generation can be postulated betweenthe two solid elements, the distance between said hypothetical plane andsaid further plane surface being equal to one-quarter of the wavelengthof said vibrations, said first solid element being composed of aquarter-wave stub adjacent the transducer and a parabolic portion joinedto said stub at an interface parallel with said hypothetical plane, thedistance from said interface to the directrix of said parabolic surfacebeing one-half of the wavelength of said vibrations.

8. A device as claimed in claim 4, in which the transducer ismagnetostrictive and has a length equal to onehalf of the wavelength ofsaid vibrations between said first plane surface and said second planesurface, the distance between said second plane surface and said furtherplane surface being equal to one-quarter of the wavelength of saidvibrations, the distance between said first plane surface and thedirectrix of said parabolic surface being equal to one-quarter of thewavelength of said vibrations.

9. A device as claimed in claim 4, in which the transducer ismagnetostrictive and has a length equal to onehalf of the wavelength ofsaid vibrations between said first plane surface and said second planesurface, the distance between said second plane surface and said furtherplane surface being equal to one-quarter of the wavelength of saidvibrations, said first solid element being composed of a quarter-wavestub adjacent the transducer and a parabolic portion joined to said stubat an interface parallel with said hypothetical plane, the distance fromsaid interface to the directrix of said parabolic surface being one-halfof the wavelength of said vibrations.

10. A device as claimed in claim 1, in which said second solid elementhas a convex parabolic surface of which the parabolic axis is normal tosaid second plane surface, the directrix of said last-mentionedparabolic surface being parallel to said second plane surface, apassageway in said second solid element through which fluid may betransmitted, said last-mentioned passageway containing the focus of saidlast-mentioned parabolic surface.

11. A device as claimed in claim 10, in which the transducer iselectrostrictive and is thin compared with said solid elements, wherebya hypothetical plane of vibration generation can be postulated betweenthe two solid elements, the distance between said hypothetical plane andthe directrix of said last-mentioned parabolic surface being equal toone-quarter of the wavelength of said vibrations, the distance betweensaid hypothetical plane and the directrix of said first-mentionedparabolic surface being equal to one-quarter of the wavelength of saidvibrations.

12. A device as claimed in claim 10, in which the transducer iselectrostrictive and is thin compared with said solid elements, wherebya hypothetical plane of vibration generation can be postulated betweenthe two solid elements, said first solid element being composed of afirst quarter-wave stub adjacent the transducer and a first parabolicportion joined to said first quarter-wave stub at a first interfaceparallel with said hypothetical plane, the distance from said firstinterface to the directrix of said first-mentioned parabolic surfacebeing one-half of the wavelength of said vibrations, said second solidelement being composed of a second quarter-wave stub adjacent thetransducer and a second parabolic portion joined to said secondquarter-wave stub at a second interface parallel with said hypotheticalplane, the distance from said second interface to the directrix of saidlastmentioned parabolic surface being one-half of the wavelength of saidvibrations.

13. A device as claimed in claim 10, in which the transducer ismagnetostrictive and has a length equal to onehalf of the wavelength ofsaid vibrations between said first plane surface and said second planesurface, the distance between said first plane surface and the directrixof said first-mentioned parabolic surface being equal to one-quarter ofthe wavelength of said vibrations, the distance be tween said secondplane surface and the directrix of said quarter of the wavelength ofsaid vibrations.

14. A device as claimed in claim 10, in which the transducer ismagnetostrictive and has a length equal to onehalf of the wavelength ofsaid vibrations between said first plane surface and said second planesurface, said first solid element being composed of a first quarter-wavestub adjacent the transducer and a first parabolic portion joined tosaid stub at a first interface parallel with said hypothetical plane,the distance from said first interface to the directrix of saidfirst-mentioned parabolic surface being one-half of the Wavelength ofsaid vibrations, said second solid element being composed of a secondquarter-Wave stub and a second parabolic portion joined to said secondquarter-wave stub at a second interface parallel with said hypotheticalplane, the distance from said second interface References Cited UNITEDSTATES PATENTS 10 MILTON O. HIRSHFIELD, Primary Examiner B. A. REYNOLDS,Assistant Examiner US. Cl. X.R.

to the directrix of said last-mentioned parabolic surface 15 59 0 beingone-half of the wavelength of said vibrations.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,524,083 August 11, 1970 Anthony J. Last et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 7, lines 50 to 55, the formula should appear as shown below:

Signed and sealed this 23rd day of February 1971.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

